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Pharmacy Science And Calculations Sessions 1-6 Practice Test

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A dilution will produce 200 mL of a solution at 3 mg/mL using a stock solution of 10 mg/mL. How many milliliters of stock solution are needed?
Correct Answer:
60 mL
Explanation:
The key idea is that the total amount of drug stays the same when you dilute. Use C1V1 = C2V2 (equivalently, final concentration times final volume equals the amount of drug you need). First, find how much drug is required in the final solution: 3 mg/mL × 200 mL = 600 mg of drug. From the stock solution, you have 10 mg of drug per mL. To get 600 mg, you need 600 mg ÷ 10 mg/mL = 60 mL of stock solution. So, you would use 60 mL of the stock solution and dilute it up to 200 mL total. This gives a final concentration of 600 mg/200 mL = 3 mg/mL.

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